1. Field of the Invention
The present invention relates to a liquid crystal display device that makes a liquid crystal display panel of, for example, an optically compensated bend (OCB) mode to periodically perform video signal display and non-video signal display.
2. Description of the Related Art
Flat-panel display devices realized by liquid crystal display devices are widely used to display images in computers, car navigation systems, TV receivers and similar equipment. A typical liquid crystal display device utilizes a liquid crystal display panel that includes a matrix array of liquid crystal pixels, and a display panel control circuit that controls the display panel. The liquid crystal display panel has a structure in which a liquid crystal layer is held between an array substrate and a counter-substrate.
The array substrate includes a plurality of pixel electrodes arrayed substantially in a matrix, a plurality of gate lines arranged along the rows of pixel electrodes, respectively, a plurality of source lines arranged along the columns of pixel electrodes, and a plurality of switching elements arranged near intersections between the gate lines and the source lines. Each of the switching elements is composed of, e.g., a thin-film transistor (TFT). The switching element is turned on when one associated gate line is driven, thereby applying a potential of one associated source line to one associated pixel electrode. The counter-substrate includes a common electrode which is opposed to the pixel electrodes disposed on the array substrate. A pair of one pixel electrode and the common electrode, together with a pixel region that is a part of the liquid crystal layer held between these electrodes constitute a pixel and control the alignment of liquid crystal molecules in the pixel region by an electric field created according to a liquid crystal drive voltage, which is a difference in potential between the pixel electrode and common electrode. The display panel control circuit includes a gate driver that drives the gate lines as a vertical driving circuit, a source driver that drives the source lines as a horizontal driving circuit, and a timing controller that controls the operation timings of the gate driver and source driver on the basis of image data and sync signals supplied externally.
In liquid crystal display devices for TV receivers, that principally display moving images, the introduction of a liquid crystal display panel of an OCB mode in which liquid crystal molecules exhibit good responsivity (see Jpn. Pat. Appln. KOKAI Publication No. 2002-202491), has been studied. Before supply of power, liquid crystal molecules are aligned in a splay alignment in which most of the molecules are laid down by alignment films that are provided on the pixel electrode and the common electrode and are rubbed in mutually parallel directions. In this liquid crystal display panel, a display operation is performed after the splay alignment is transitioned to a bend alignment in an initialization process by a relatively intense electric field that is applied upon supply of power.
The reason why the liquid crystal molecules are aligned in the splay alignment before supply of power is that the splay alignment is more stable than the bend alignment in terms of energy in a voltage-non-applied state of a liquid crystal drive voltage. Even if the liquid crystal molecules once transition to the bend alignment, reverse transition from the bend alignment to the splay alignment tends to occur if a voltage-non-applied state, or a voltage-applied state of a voltage not greater than a level at which energy of the splay alignment is balanced with energy of the bend alignment, continues for a long time. In the splay alignment, abnormality in display may occur since the viewing angle characteristics of the splay alignment are sharply different from those of the bend alignment.
In the prior art, in order to prevent the reverse transition from the bend alignment to the splay alignment, such a driving method is adopted that a high voltage is applied to the OCB liquid crystal pixel, for example, in a part of a frame period within which a single-frame image is displayed. In a normally-white liquid crystal display panel, this voltage corresponds to a black-display voltage, so this driving method is called “black insertion driving.”
In a conventional black insertion driving method, the gate liens Y1 to Ym shown in FIG. 6 are scanned two times for black insertion writing and video signal writing in two vertical scanning periods by the gate driver. The gate driver includes a shift register that receives a vertical start pulse STV supplied every vertical scanning period, and shifts the vertical start pulse STV in sync with a vertical clock signal CKV. The gate driver sequentially selects and drives the gate lines Y1 to Ym based on the shifted position of the vertical start pulse. The source lines X1 to Xn are driven in parallel by the source driver while each of the gate lines Y1 to Ym is kept driven. The source driver includes a shift register that receives a horizontal start pulse STH supplied every horizontal scanning period, and shifts the horizontal start pulse STH in sync with a horizontal clock signal CKH. To the source driver, items of pixel data (video signal S or black display signal B) for one row (a horizontal line) are supplied in series during the horizontal scanning period. The source driver captures the pixel data items based on the shifted position of the horizontal start pulse STH, converts them into pixel voltages, and outputs the pixel voltages in parallel toward the source lines X1 to Xn, in response to a latch output pulse LT.
However, in the black insertion driving method, the pixels from the first row to the last row perform video signal display with pixel voltages which are sequentially written and held for one vertical scanning period, and perform black insertion display (non-video signal display) with pixel voltages which are sequentially written and held for one vertical scanning period. The video signal S falls within a range from the black display level of the minimum gradation and the white display level of the maximum gradation. If the white display level is maintained for all the pixels, the following display is repeated. That is, the black display area is increased from the upper end of the display panel to the lower end thereof upon the initiation of black insertion write scanning, and then, the white display area is increased from the upper end of the display panel to the lower end thereof upon the initiation of video signal write scanning. Thus, the viewer of the display panel recognizes the variation in luminance occurring on the panel as flicker.